I second this recommendation. I don't use the Disroot app on my phone, but they also offer a bunch of other great apps beyond emails in case you are interested. You can check them on their website. They're all FOSS and free to use. Most of them don't even require an account.

Thank you for the suggestion, unfortunately as this is for business, this one probably won't work for us.

They're also a Dutch non-profit organization, and both their services and their infra are all developed/maintained in the open using exclusively open-source projects.